1. Field of the Invention
The present invention relates to a microwave plasma processing apparatus utilized for fabrication processes of semiconductor devices, and particularly to a microwave plasma processing apparatus which has improved productivity with stable performance for a long period of time.
2. Description of the Related Art
An improved type of a microwave plasma processing apparatus has been disclosed in the U.S. Pat. No. 4,512,868 issued on Apr. 23, 1985 to Fujimura et. al. (same as Japanese Unexamined Patent Publication SHO 62-16424). The apparatus has a specific structure that a reactive gas is introduced into a plasma generation chamber, microwave power is introduced therein, the plasma which comprises active species is generated in the reactive gas, the active species are introduced into a separate reaction chamber arranged adjacent to the plasma generation chamber, and a substrate is disposed and processed in the reaction chamber. An outline of the disclosure will be explained with reference to FIG. 1.
FIG. 1 is a schematic cross section of the microwave plasma processing apparatus. A microwave generator 1 supplies microwave power which is transmitted into a reactor 3 through a waveguide 2 and a window 4 which seals the reactor 3 vacuum tight and makes the microwave power penetrate therethrough. The reactive gas including at least oxygen (O.sub.2) flows into the reactor 3 through an inlet pipe 5 as shown by an arrow a. In the reactor 3, a plasma shield plate 6 made of aluminum is provided, dividing the reactor 3 into the plasma generation chamber 3a and the reaction chamber 3b. The plasma shield plate 6, which is sometimes called a shower head, has a plurality of small holes 6a. A substrate 7 is arranged on a stage 8 in the reaction chamber 3b. The gas after reaction is exhausted through an exhaust pipe 3c as shown by an arrow b.
The microwave power of a few giga Hz in frequency excites the reactive gas within the plasma generation chamber 3a and generates plasma comprising active species. The uniformly diffused plasma in the plasma generation chamber 3a flows into the reaction chamber 3b through small holes 6a as shown by an arrow c. However, most of the charged particles such as ions in the plasma remain in the plasma generation chamber 3a. The neutral active species flow into the reaction chamber 3b with uniform distribution.
A resist layer coated on a surface of the substrate 7 reacts with the active species, resulting in performing uniform etching or ashing processes onto the entire surface of the substrate 7.
For example, a mixed gas of oxygen(O.sub.2) and carbon tetrafluoride (CF.sub.4) in the ratio of 8:2 is used as the reactive gas and the microwave power of about 0.4 KW and 2.45 GHz is applied into the plasma generation chamber 3a. The reaction chamber 3b is evacuated at about 0.3 Torr. A good result of ashing process for the resist layer has been obtained.
Another improvement of a microwave plasma processing apparatus has been disclosed in the U.S. Pat. No. 4718976 issued on Jan. 12, 1988 to Fujimura (same as Japanese Allowed Patent Publication SHO 62-1534). An outline of the structure thereof is shown in FIG. 2. The same reference numerals designate the same or similar parts in the drawing. A plasma generation chamber 3a is separated from a reaction chamber 3b in the similar way as shown in FIG. 1, however, active species comprised in the plasma are introduced into the reaction chamber 3b through a flow rate control means 11a (conductance controller).
In FIG. 2, the conductance controller 11a is provided at the center of metallic, for example, aluminum plasma shield plate 11 which divides reactor 3 into two sections of the plasma generation chamber 3a and reaction chamber 3b. A circular gas diffusion plate 12 is arranged in the reaction chamber 3b, and it has dimension of a diameter d and a predetermined distance h from the plasma shield plate 11 such that the plasma introduced through the conductance controller 11a can diffuse outwardly and uniformly after collision with the gas diffusion plate 12. And further, a funnel-shaped reflector 13 is provided outside the gas diffusion plate 12 and upon a substrate 7, and the reflector 13 works to make the plasma direct onto the substrate 7.
The reactive gas is introduced into the plasma generation chamber 3a from an inlet pipe 5 as shown by an arrow a'. The reactive gas is excited by microwave power, and the generated plasma flows into the reaction chamber 3b passing through the conductance controller 11a and further diffuses onto the substrate 7 along a path shown by an arrow c'
When the diameter d of the gas diffusion plate 12 and the distance h from the plasma shield plate 11 are set at optimum values, satisfactory results can be obtained.
Further the following patent applications which are related to a microwave plasma processing apparatus have been filed by Fujimura et. al.
U.S. application Ser. No. 802,332 filed on Nov. 27, 1985 (same as Japanese Unexamined Patent Publication SHO 61-131454).
U.S. application Ser. Appl. No. 24,070 filed on Mar. 10, 1987 (same as Japanese Unexamined Patent Publication SHO 62-213126).
The above two applications are related with a structure for a microwave window which partitions a plasma generation chamber from a microwave waveguide, and the details thereof are omitted herein.
Throughout all references cited above, etching or ashing processes are carried out using a reactive gas including oxygen. Because the plasma shield plate 6 or 11 shown in FIGS. 1 and 2 respectively is made of aluminum, the exposed surface of the plasma shield plate 6 to the plasma generation chamber 3a reacts with the active species of the plasma, resulting in reducing the active species.
As a result, a processing rate decreases gradually with the utilization period of the apparatus compared with the data obtained at an initial stage when the plasma shield plate is covered with a natural oxide film only.
In an experiment, wherein a reactive gas of O.sub.2 +CF.sub.4 (8% CF.sub.4 content) is used and microwave power of 1.5 KW at 2.45 GHz is applied, an ashing rate of about 1.5 .mu.m/min has been obtained for a resist layer on a substrate at an initial stage when the plasma shield plate has a natural oxide film only. However, after utilization of the apparatus for about three months, eight hour operation per day, the ashing rate decreases to 5000 .ANG./min, which is about 1/3 of the data at the initial stage.
Such phenomenon is observed even in a case when the aluminum surface of the plasma shield plate is beforehand processed with alumite treatment.
The existing microwave plasma processing apparatus has a drawback in that processing capability in a unit time gradually deteriorates during utilization thereof.